1. Field of the Invention
The present invention relates to a structure of a heating section in a heating apparatus and, more particularly, it relates to an improvement of those portions at which heating elements for heating a process tube in a CVD apparatus, for example, are connected to terminals.
The heater formed by coiling resistance heating elements (which will be hereinafter referred to as "coil heater") is used at the heating section in various heating apparatuses. Generally, a coil heater having a large diameter and a large heat capacity is used for a high temperature heating apparatus but it is small in diameter and heat capacity is used for the high temperature heating apparatus. The highest temperature allowed particularly in the CVD apparatus, for example, is 1000.degree. C. or more or less and the coil heater formed by an element of FeCrAl alloy whose diameter is 2 mm is therefore used for the CVD apparatus.
In the case of the CVD apparatus, for example, ON/OFF of current supply are frequently repeated to successively process batches of plural semiconductor wafers. The coil heater of small diameter is therefore likely to be broken because it is expanded and contracted by the heating cycles. This causes its life to be shortened. The breakdown of a coil heater is frequently caused particularly at connected portions of a terminal and the coil heater because a temperature gradient is large at the terminal, and the terminal is so shaped as to allow local stress concentration on them.
Japanese Utility Model Publication Sho 53-40761 discloses a heating apparatus wherein a buffer arm is attached to the terminal, through which current is supplied to the coil heater, to absorb the strain of the terminal caused by the heat expansion of the coil heater. In the case of this apparatus, however, heat deformation caused at the terminal can be absorbed by the buffer arm but those caused at the other portions cannot be absorbed by it. The coil heater is therefore likely to be fatigued not to fully prevent breakdown of coil heater.
Japanese Patent Disclosure Sho 58-31282 discloses an electric furnace having a heat loss preventing block arranged between an opening through which the terminal for supplying current to the coil heater and a quartz pipe. In the case of this electric furnace, the heat loss preventing block is located between the opening and the quartz pipe to prevent heat loss through the opening of the furnace body, so that a length of the uniform temperature zone in the heating apparatus can be made long. However, the opening and its vicinity are made high in temperature because of heat energy radiated through the heated quartz pipe and heat energy transmitted from the heat loss preventing block. When the inside of the furnace is heated to about 1000.degree. C., for example, the outer wall of the furnace body and the opening and its vicinity thereof are heated to a temperature higher than 300.degree. C. The cooling fan and the heat exchanger must be therefore added to prevent the whole of the furnace from being excessively heated, thereby causing the whole of the furnace to be large-sized.
Japanese Utility Model Publication Sho 53-48589 discloses a heating apparatus having an intermediate casing in which a heat element is arranged an outer casing located outside the intermediate casing, and a means for supplying cooling air between the intermediate and the outer casing. This apparatus has no specific cooling means at the heating section. When the inside of the apparatus is heated to about 1000.degree. C., for example, therefore, the temperature of the heater base and the outer wall for the horizontal-type pipe-shaped heaters are elevated to about 300.degree. C. In order to eliminate heat from these components, the three horizontal-type pipe-shaped heaters are thus enclosed by an inner casing and cooling air is supplied along the outer face of the inner casing. As the result, the heating apparatus becomes complicated and large-sized as a whole.
Japanese Utility Model Publication Sho 60-31259 discloses a heating apparatus wherein an electrically insulating bushing is interposed between a cartridge heater and a support to prevent current from being leaked from the power source to ground through a heater case. It is needed in this apparatus that insulating members such as the insulating plate and bushing are used at various portions to insulate the cartridge heater from the support. This makes the attaching of the cartridge heater complicated. Further, when the heater is to be made smaller-sized, the insulating member must be attached to the high temperature portion of the heater case. This makes it impossible to obtain a desired insulation resistance, so that current leakage cannot be fully prevented.
Japanese Patent Disclosure Hei 1-114032 discloses a heating apparatus wherein the winding density of a coil heater is made higher at a center zone in the horizontal-type ring-shaped furnace than those at both side zones thereof. When the inner diameter of the heating section in the apparatus is changed, the length of the process tube inserted into the apparatus is changed, or a heat shielding means at the furnace opening in the apparatus is changed, it is needed in the case of this apparatus that the winding density of the coil heater at the center zone is optionally set in the range of 5-15%.
Japanese Utility Model Disclosure Sho 61-89800 discloses a heating apparatus wherein a coil heater is held in a rectangular hole of a cylindrical member which is formed by two halves each having a groove-like recess on the inner face thereof and made of ceramics. In the case of this apparatus, several hundred sets of the coil heater support members each of which is a cylindrical member formed by the two recessed halves are provided to keep a predetermined winding pitch of the coil heater unchanged even when the coil heater is repeatedly expanded and contracted every time current supply is started and stopped. Every time the winding density of the coil heater is changed, therefore, several hundred sets of the support members whose dimension is different from that of the previous ones must be used These support members are complicated in shape and have a high cost and a long time is thus needed to make one set of the support members.
Japanese Patent Disclosure Sho 60-246582 discloses a heating apparatus of the composite type wherein a coil heater of small diameter is embedded in refractory ceramics. When a plurality of heating units each of which comprises the coil heater and the refractory ceramics are piled one upon an other, the heating apparatus can form the equally-heated section at a desired area. In order to increase the length of uniform temperature zone in the apparatus, however, a plurality of the heating zones or 10 heating zones, for example, must be provided. When temperature is independently controlled at each of the heating zones, therefore, the adjacent heating zones interact with each other and temperature thus fluctuates at each of the heating zones, thereby making it difficult to stably control temperature.
Further, a metal cover encloses the refractory ceramics and current supply terminals are attached to this metal cover. The metal cover is made of aluminium, for example. When those portions of the metal cover to which the current supply terminals are attached are made high in temperature, local stress is caused at connecting portions of the coil heater at the terminals to which ends of are connected, because of thermal expansion difference between the aluminium cover (whose thermal expansion coefficient is about 16.times.10.sup.-6 /K) and the refractory ceramics (whose thermal expansion coefficient is about 0.5.times.10.sup.-6 /K). Breakdown is thus caused at these connecting portions.